Metronidazole For InfantThe dose of Flagyl (metronidazole) your daughter received seems reasonable and is within the dosage indications for a 7-week old baby. Based on the known pharmacokinetic profile of metronidazole, it would have been eliminated from your baby’s body after about 4 days. 


Metronidazole has not been shown to increase the risk of cancer in humans, as is discussed in more detail below.

 

Flagyl (Metronidazole) Infant Dosing

Metronidazole is an anti-infective drug that kills parasites (like those that cause amebiasis) and some bacteria (but it doesn’t kill klebsiella). 


The dose for metronidazole in infants is based on the baby’s weight. The recommended dose to treat amebiasis ranges from 5.3 mg to 7.6 mg per pound that the infant weighs, 3 times a day for 7 to 10 days.


Your baby was getting 42.5 mg (1.7 mL) to 50 mg (2 mL) 3 times per day. If your baby weighed about 8-9.5 pounds, then she did receive the correct dose. 


Flagyl (Metronidazole) Metabolism In Infants

Metronidazole is metabolized extensively in the liver and it is eliminated from the body through the urine and feces.  How quickly metronidazole is cleared in infants depends on the age.


Studies show that in infants less than 8 weeks old, the half-life of metronidazole is around 18.4 hours. In infants more than 8 weeks old, the half-life is around 7-9 hours.


Half-life refers to the time it takes for 50% of a drug to be metabolized and cleared from the body. In general, it takes around 5 half-lives for a drug to be completely eliminated.


Therefore, in the majority of babies less than 8 weeks old, metronidazole would completely eliminated from the body after about 4 days. 

 

Flagyl (Metronidazole) - Cancer Risk

The studies that showed metronidazole caused cancer in rats and mice used high doses over the lifetime of the rats and mice. They did the same study with hamsters and metronidazole didn’t cause cancer. 


One study showed there were no adverse effects when rats were given high doses of metronidazole for 28 days. Studies that have been done in humans haven’t shown an increased risk of cancer. 


However, there have been anecdotal reports of breast and colon cancer in individuals diagnosed with Crohn's disease who have been treated with metronidazole at high doses for extended periods of time. Nevertheless, there has been no established cause and effect relationship.


Summary

  • Flagyl (metronidazole) is used commonly in infants for a range of indications.
  • Flagyl (metronidazole) is dosed based on the weight of the infant, around 5.3 to 7.6 mg per pound per dose, in three doses per day.
  • Flagyl (metronidazole) will take around 4 days to be completely eliminated from the body in babies less than 8 weeks old. 
  • Human studies have shown no increase in cancer risk or are at most inconclusive based on current data.